The field of the invention is that of transmitting data from a plurality of sources to a plurality of receivers via a single gateway.
The invention applies, for example, although not exclusively, to broadcasting data via satellites to terminals that are isolated and/or to which access is difficult.
A preferred field of application of the invention is that of broadcasting high bit rate data, for example in the context of the Internet.
There are very many systems for transmitting to a plurality of receivers, of course. The information to be transmitted is multiplexed in such cases.
Such systems include:
conventional time division multiplexing (TDM) whereby the data is sent in sequence using frame organization, each frame being formed of a series of time slots each allocated to one call;
frequency division multiplexing (FDM) in which each call is allocated to a respective carrier frequency; and
code division multiplexing (CDM) in which the signals of all calls are transmitted simultaneously on the same carrier, each having been multiplied beforehand by a code selected so that it is orthogonal to all the other codes (with multiplication by the same code in the receiver recovering the source signal).
The conventional TDM approach is based on using time division multiplexing on a single carrier frequency.
For improving the TDM technique consideration has been given to using two of the above techniques simultaneously to obtain a multicarrier TDM system. In this case the receiver is informed of the carrier frequency that it must use by signalling data.
FIG. 1 shows a prior art system of the above kind.
Consider the case of n calls to be transmitted each having a given bit rate d1 through dn. To transmit these calls to n receivers p carrier frequencies are provided (p being less than n).
In this case each call 111 through 11n is allocated to one of the carrier frequencies 121 through 12p for the entire call. The receiver 13 is tuned to the appropriate carrier frequency fi.
The bit rate obtained on each carrier fI is:       S    i    =            ∑              j        =        1                    n        j              ⁢                  d        _            j      
nj being the number of calls transmitted on the carrier fi. Of course, the total bit rate obtained:             ∑              i        =        1            p        ⁢          S      i        =            ∑              i        =        1            p        ⁢                  ∑                  j          =          1                          n          j                    ⁢                        d          _                j            
remains less than the potential bit rate S.
Allocating resources in the above manner is not optimal. For example, consider the particular case of a queue of n servers with a Poisson type distribution, in which case the method obeys Erlang""s law. With the same number of overall circuits and given probability of blocking, this is not favorable to server utilization with segmentation into small independent subsystems (i.e. subsystems using a small number of servers).
The prior art is also represented by document FR 2 737 366 which concerns a device for transmitting data from a plurality of sources to a plurality of receivers via a single gateway using a plurality of sub-carriers to transmit packets of data. However, the above device does not provide a global analysis of all the data to be transmitted to maximize the use of available transmission resources.
Finally, document EP 0 667 695 describes a data transmission method in which the data to be exchanged is distributed between a plurality of carriers transmitted in parallel, each radio station being fixedly allocated a particular number of carriers. This fixed allocation of frequencies rules out optimizing the use of available transmission resources.
An object of the invention is to mitigate the above drawbacks of the prior art.
To be more precise, an object of the invention is to provide a method of transmitting on a plurality of carrier frequencies suited to transmitting a plurality of streams of data to separate receivers which maximizes the use of available transmission resources.
In particular, an object of the invention is to provide a method of the above kind suitable for transmitting data signals organized into blocks or packets, for example data broadcast over the Internet.
Another object of the invention is to provide a method of the above kind that does not significantly increase the complexity and/or the unit cost of the corresponding terminals.
The above objects, and others that become apparent hereinafter, are achieved according to the invention by a method of transmitting data from a plurality of sources to a plurality of receivers via a single gateway using at least two separate data transmission media, said receivers being able to receive selectively data transmitted on any of said transmission media, the method being characterized in that it assures dynamic distribution of said data to be transmitted to said transmission media in accordance with a global analysis of all of said data to be transmitted, signalling data regularly specifying to each of said receivers the transmission medium or media it must use.
In other words, the invention assures global dynamic allocation, optimizing use of the radio resource.
In one advantageous embodiment of the invention in the context of multicarrier transmission said transmission media are carrier frequencies.
In another embodiment of the invention in the context of CDMA transmission said transmission media can be codes.
Said data is advantageously organized into packets of data, each packet being sent on a particular transmission medium.
The destination information specifying the receiver to which a data packet is addressed can be transmitted:
in said packet itself in the form of a packet header; or
by means of a transmission medium dedicated to transmitting destination information.
These can be packets conforming to the Internet data transfer protocol.
The invention is particularly advantageous in situations in which the number of transmission media is very much lower than the number of sources (for example one for every few dozen).
The invention also concerns a transmitter for a system for transmitting data from a plurality of sources to a plurality of receivers, the transmitter being characterized in that it comprises:
transmission means using at least two separate data transmission media, said receivers being able to receive selectively data transmitted on any one of said transmission media;
means for dynamically distributing said data to be transmitted to said transmission media in accordance with a global analysis of all of said data to be transmitted; and
means for generating signalling data regularly specifying to each of said receivers the transmission medium or media it must use.
A transmitter of the above kind advantageously further comprises means for temporarily storing blocks of data addressed to one of said receivers.
This optimizes it for transferring packets of data.
The invention further concerns the transmitter/receiver terminals used in the context of the method described hereinabove. The terminals include means for receiving data transmitted in response to a request using a transmission method employing at least two separate data transmission media, said terminal being adapted to receive selectively data transmitted on any one of said transmission media and assuring dynamic distribution of said data to be transmitted to said transmission media in accordance with a global analysis of all of said data to be transmitted, signalling data regularly specifying to said terminal the transmission medium or media that it is to use.